This invention relates to articles having a ceramic surface and, more particularly, to a cementitious ceramic coating having a controllable surface reflectance and texture.
Ceramic coatings are sometimes used to protect and/or insulate substrate articles that would otherwise be subject to mechanical or thermal damage. Ceramics are typically hard and resistant to abrasion damage and the like. They also can have a low coefficient of thermal conductivity and act as insulators for the underlying structure. At their present state of development, ceramics are not widely used as the underlying structural components because of their low ductility and fracture toughness.
When applied as coatings, cementitious ceramics typically have uncontrolled, but usually poor, reflection surface characteristics. For example, such ceramic coatings may be applied by gunning and curing techniques, which result in a relatively rough coating surface that has poor reflection properties. Ceramic coatings may be applied by plasma spraying and related techniques, again producing a surface that is largely uncontrolled. The term xe2x80x9cuncontrolledxe2x80x9d is used here to mean that little if any independent control can be exerted over the character of the surface, to provide a selectable type of surface reflectance and texture.
Ceramic coatings can also be made by physical vapor deposition (PVD) techniques such as sputtering or thermal evaporation. These coatings are typically very thin (i.e., less than one micrometer in thickness). The coatings can be made to be glossy and highly reflective under some deposition conditions, but they follow the underlying surface topography and are not thick enough to form a three-dimensional textured surface.
Reflective ceramic surfaces can be formed with glazing techniques such as used on dinnerware. Finely divided glass, termed glass frit, is sprayed onto the surface of a ceramic substrate. The ceramic and glass frit are heated to a high temperature to cause the glass frit to melt and flow, creating a smooth, glazed ceramic surface coating which follows the contour of the ceramic substrate. The glassy surface coating is not, however, xe2x80x9csetxe2x80x9d in the manner of a cementitious coating, and will reflow if the glassy coating is heated above its glass transition temperature.
The surface finish of any material, including a ceramic coating, may be of importance in many applications. The surface smoothness influences properties such as aerodynamic resistance, boundary layer thickness, aerothermal heating, and the like. The ability of the surface to reflect light determines, in part, its resistance to damage from impinging high-intensity light beams. Various techniques are available for controlling the surface character of metals and polymers, but, as discussed, it has been difficult to selectively control the surface finish of cementitious ceramic coatings. Thus, for example, it has not been possible to apply a smooth, highly reflective cementitious ceramic coating to a metallic, ceramic, or polymeric substrate, with a controllable surface texture. Such coatings, if available, would be valuable tools in controlling surface mechanical and thermal properties.
There is therefore a need for a technique for producing a controllably reflective surface on cementitious ceramics, and particularly on cementitious ceramic coatings. Such a technique desirably permits the coatings to be applied to a variety of substrates and with a variety of surface textures, while simultaneously yielding a highly reflective coating. The present invention fulfills this need, and further provides related advantages.
The present invention provides a method for preparing a controllably reflective cementitious ceramic surface, typically in the form of a coating, and an article having such a ceramic surface. The approach allows all processing to be completed at intermediate temperatures, but the ceramic may be used to much higher temperatures in service without loss of the desirable surface properties. The relatively low processing temperature also permits relatively inexpensive tooling and heating equipment to be used. Thus, a reflective cementitious coating can be applied to many substrates without removing the substrates from their underlying structure, so that field installations and repairs are practical. In a preferred form, the surface of the ceramic is highly reflective of visible light. A surface texture can be applied to the surface of the ceramic, without sacrificing the reflective finish.
In accordance with the invention, a method for preparing an article having a ceramic surface comprises the steps of providing an article having a surface to be coated and preparing an aqueous mixture of a source of a reactive phosphate ion and a nonmetallic ceramic form of a cation reactive with phosphate ion to form a ceramic phosphate. The mixture is contacted to the surface of the article, and a mechanical overpressure is applied to the mixture at the surface of the article. The mixture is set, typically just after the application of the overpressure, and thereafter cured without any overpressure.
In one preferred application, the source of reactive phosphate ion is phosphoric acid, and the nonmetallic ceramic form of a cation is a mixture of alumina powder and cordierite powder. The mechanical overpressure is applied either with a smooth tool or an intentionally textured tool to produce a controllably textured, preselected final surface profile in the coating. The coating may be applied to a wide variety of substrate articles, such as, for example, metals, metal-matrix composites, ceramics, ceramic-matrix composites, organic materials, and organic-matrix composites.
In another aspect of the invention, a method for preparing an article having a ceramic surface comprises the steps of preparing an aqueous mixture of a source of a reactive phosphate ion and a nonmetallic ceramic form of a cation reactive with phosphate ion to form a ceramic phosphate and placing the mixture at the surface of an article. A mechanical overpressure is applied to the mixture at the surface of the article, and the mixture is set and cured.
The approach of the invention provides an advance in the art of ceramic materials. Bulk and coated cementitious ceramics with controllably reflective surfaces can be prepared with the use of no more than intermediate processing temperatures. The surface can also be textured, if desired, in the same processing. The ceramics can be used to much higher temperatures without loss of the surface properties. Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention.